To be fair, neither is exactly terminology familiar with the average user, who I think they're trying to reach out to here. Both are clever, but known mostly by avid Internet users or nerds.
I think ToS;DR is less nerdy. "Grok" makes me visualize gray-bearded Unix programmers, but young internet-savvy people of all stripes know what TL;DR means. Didn't 4chan popularize that abbreviation?
Certainly. But I think people who care what provisions the TOS of a service contain are the exception and not the average person.
I think one of the goals with providing brief, easy to read summaries can (and should) be expanding the number of people who care what is in the TOS.